Thanks "the Tin Man" -- you are correct, that section under "readline" was exactly what helped me. I'm finally 1.9.2 after hours of trying to install on a fresh version of Mac OSX Mountain Lion and 1.9.2.
–
rsturimMar 2 '13 at 7:02

I'm pasting the output of 'rvm notes' here as this matters directly. You need at least Xcode 3.2.1 (1613). Please note that Snow Leopard was the firs to have this revision so you'll need to grab the updated Xcode.

∴ rvm notes

Notes for Darwin ( Mac OS X )
For Snow Leopard be sure to have Xcode Tools Version 3.2.1 (1613) or later
You should download the latest Xcode tools from developer.apple.com.
(This is since the dvd install for Snow Leopard contained bugs).

If you intend on installing MacRuby you must install LLVM first.
If you intend on installing JRuby you must install the JDK.
If you intend on installing IronRuby you must install Mono
(version 2.6 or greater is recommended).

NOTE:
As of 1.8.0 RVM once again loads .rvmrc files, by default, after asking your
permission to trust it of course. If you do not wish to be enabled, simply set:
export rvm_project_rvmrc=0
Within either your /etc/rvmrc or $HOME/.rvmrc file. This will turn off the
cd/pushd hooks when sourcing the rvm() function into your shell.

Example: echo 'export rvm_project_rvmrc=0' >> $HOME/.rvmrc;
(Then close the current shell and open a new one.)

As of this writing the current RVM is 1.8.3, the required Xcode is minimally 3.2.1, and you can use either the readline in Homebrew, RVM's 'rvm pkg install readline' or Lion's built-in readline (though there have been significant reports of that failing).